An optical system of an electronic camera, typically including general video cameras, digital still cameras and the like, comprises an integrated optical system, an IR(Infrared Ray)-cut filter, an optical low pass filter, and an imaging device, such as a CCD (Charge Coupled Device), a MOS (Metal Oxide Semiconductor) or the like, which are placed sequentially along an optical axis with the closest to the subject listed first (see, for example, JP 2000-209510A).
An imaging device as described above has sensitivity characteristics that respond to light in a wider wavelength band than that visible to the human eye (visible light) as shown in FIG. 13. In other words, the imaging device responds to infrared or ultraviolet light as well as visible light. It should be noted that FIG. 13(a) shows the sensitivity characteristics of the human eye, while FIG. 13(b) shows the sensitivity characteristics of a general CCD.
As can be seen from these figures, the human eye responds to light in the wavelength range of about 400 to 620 nm in the dark and light in the wavelength range of about 420 to 700 nm in the light. In contrast, the CCD responds to light in the wavelength range of 400 to 700 nm as well as light having a wavelength of less than 400 nm and light having a wavelength of more than 700 nm.
Therefore, in an imaging device as described in JP 2000-209510A, an IR-cut filter is provided in addition to a CCD to prevent infrared light from reaching the imaging device, thereby obtaining a captured image which is more approximate to that perceived by the human eye.
Conventional examples of such an IR-cut filter include an infrared absorbing glass which is transparent to visible light and absorbs infrared light, an infrared cut coat which is transparent to visible light and reflects infrared light, and the like.
An example of an infrared absorbing glass is blue glass in which a pigment, such as copper ion or the like, is dispersed.
An example of an infrared cut coat is a dielectric multilayer film in which a high refractive index material, such as TiO2, ZrO2, Ta2O5, Nb2O5 or the like, and a low refractive index material, such as SiO2, MgF2 or the like, are alternately layered up to several tens of layers on a transparent plate.
The infrared absorbing glass and infrared cut coat will be hereinafter described with reference to FIG. 14. It should be noted that FIG. 14(a) shows transmittance characteristics when an infrared absorbing glass is used in an imaging device, while FIG. 14(b) shows transmittance characteristics when an infrared cut coat is used.
Firstly, as shown in FIG. 14(a), when an infrared absorbing glass is used, “characteristics in which transmittance decreases gradually” can be obtained from a visible region to an infrared region, which are approximate to the sensitivity characteristics of the human eye.
However, when an infrared absorbing glass is used, it is difficult to adjust a point, where the transmittance is substantially 0%, to 700 nm. In the case of the infrared absorbing glass of FIG. 14(a), light at about 750 nm is also transmitted. In other words, infrared light is not completely cut out, so that the infrared image is captured by an imaging device.
Next, as shown in FIG. 14(b), when an infrared cut coat is used, “characteristics in which transmittance decreases sharply” can be obtained. Therefore, it is easy to adjust a point, where the transmittance is substantially 0%, to 700 nm.
However, in the case where transmittance changes in such a sharp manner, an image captured by an imaging device has a color different from that perceived by the human eye.
Therefore, the present invention is provided in order to solve the above-described problems. An object of the present invention is to provide a ray cut filter which prevents transmittance from changing sharply in a predetermined wavelength band (e.g., a visible region) to obtain transmittance characteristics approximate to those perceived by the human eye.